1. Field of the Invention
The present invention relates to a plasma jet ignition system.
2. Description of the Prior Art
Most internal combustion engines have a spark ignition system for igniting the combustion chamber charge.
However, spark ignition systems have a problem in that the spark produced across the spark plug electrodes frequently fails to ignite the combustion charge.
In order to solve this problem and to provide improved ignition performance, a plasma jet ignition system is proposed wherein a plasma ignition current from a low tension electric power source is supplied to the spark plugs for propagating a plasma jet between spark plug electrodes, thereby improving the ignition performance.
As shown in FIG. 1, the conventional plasma jet ignition systems are provided with a high tension electric power source 1 for supplying a spark ignition current to spark plugs P in a conventional manner, and a low tension electric power source 2 for supplying a low tension plasma ignition current having a low voltage (3kV) to the spark plugs P each time an ordinary spark discharge occurs.
The high tension electric power source 1 is constructed similarly to conventional spark ignition system so that a high tension voltage is generated at a secondary winding of an ignition coil when contact points open at each time of ignition. This high tension secondary current is then delivered, in turn, to spark plugs P through a distributor 3, thereby causing spark discharge between spark plug electrodes.
The low tension electric power source 2 includes step-up means for producing a voltage of 3kV, such as a DC-DC converter.
The plasma ignition energy produced by this low tension electric power source 2 is accumulated in a capacitor C, then dumped into one of the spark plugs P. More specifically, the electric charge of the capacitor C is always applied to the spark plugs P through the diodes D, and it is discharged through one of spark plugs P to which the spark ignition current is supplied. The charge on the capacitor C is automatically discharged through the spark plug electrodes due to the dielectric breakdown between the spark plug electrodes caused by ordinary spark discharge. As a result of this selective discharge of the plasma ignition energy, the diodes D can be connected directly to the spark plugs P without passing through the distributor 3.
In short, in this plasma ignition system the plasma ignition energy is directly applied to the spark plugs P and the plasma ignition energy is discharged by dielectric breakdown across the spark plug electrodes of the spark plug to which the high tension spark ignition current is supplied.
However, in a case of the plasma ignition system constructed as above, there is a problem in that the electric energy of the capacitor C is often discharged prior to the optimum ignition timing, which is often referred to as an "irregular discharge".
The irregular discharge is due to a reduction in the dielectric breakdown voltage across the spark plug electrodes. The dielectric breakdown voltage varies as a function of the pressure within the combustion chamber; it has a minimum value during intake stroke of the engine. Therefore, a discharge of the plasma ignition energy may frequently occur prior to the spark discharge of the high tension ignition current.
Once this irregular discharge occurs, the charging of the capacitor C becomes insufficient at the optimum ignition timing, rendering it impossible to propagate the plasma jet by the plasma ignition current. Moreover, if a spark caused by an irregular discharge is produced during the induction stroke, a backfire may result.